A small, high-efficiency dividing circuit and antenna duplexer used in a mobile communication device such as a cellular phone have been recently demanded. A conventional circuit block diagram of a radio frequency (RF) component such as the mobile communication device is shown in FIG. 11. FIG. 11 shows an antenna duplexer in a composite terminal for a GSM system at 900 MHz band, which is used at a cellular phone in Europe, and a DCS system at 1.8 GHz band.
As shown in FIG. 11, the RF component includes antenna port 61, transmitting ports 62, 64, receiving ports 63, 65, control terminals 66, 67, dividing circuit 68, switching circuits 69, 70 and low pass filters 71, 78. Dividing circuit 68, being generally formed of a low pass filter and high pass filter, divides a received signal into a GSM band and a DCS band. Switching circuits 69, 70 switch between a transmitting signal and a received signal at respective bands.
The receiving ports are connected with respective band-pass filters, e.g., surface acoustic wave (SAW) filters 73, 74, for limiting receiving bands. As shown in FIG. 12, antenna duplexer 76 and SAW filter 77 are mounted individually on board 75 of the cellular phone, thus requiring a large mounting area.
The cellular phone is recently required, for a further convenience, to be small as well as to have a multi-band function and more functions. An antenna duplexer responsive to the requirement is accordingly demanded.
The above-mentioned antenna duplexer, since including the antenna duplexer and band-pass filter mounted individually on the board, requires a land and a space for connecting the duplexer with the filter. This makes a communication device including the antenna duplexer be hardly small. Further, since the individual components are connected with each other, a signal is transmitted on the board. This creates a loss of the signal on the board, thus reducing a receiving sensitivity of the communication device.
The SAW filter used for the band-pass filter for limiting the receiving band is influenced by a parasitic inductance produced between a ground and a SAW resonator connected with the ground. That is, the inductance shifts an attenuation pole at a lower frequency in an attenuation band toward a lower frequency, and decreases an amount of attenuation at the pole. The SAW filter, when being integrated with the antenna-duplexer formed in a laminated body, has a grounding port thereof coupled with a grounding terminal via a leading line and a via-conductor in the laminated body. As a result, a parasitic inductance of the leading line reduces an attenuation amount at a lower frequency of the SAW filter.